It is known in the art to purify a gas stream by separating oxygen from the gas stream with the use of electrically driven oxygen ion transport elements. Such elements are provided with a composite structure that includes an electrolyte layer to conduct oxygen ions that is located between two electrode layers to apply an electrical potential across the electrolyte. The electrode layers are porous and can have sublayers while the electrolyte is an air-tight dense layer. The resultant composite structure can be in the form of a tube in which the oxygen containing feed is fed to the inside of the tube and the separated oxygen is collected on the outside of the tube and then dissipated. The reverse is possible and oxygen can be fed to the outside of the tube and the permeated oxygen collected on the inside of the tube. Other forms are possible, for example, flat plates and honeycomb-like structures.
The electrolyte layer is formed of an ionic conductor that is capable of conducting oxygen ions when subjected to an elevated temperature and an electrical potential is applied to the electrode layers. Under such circumstances, the oxygen ions will ionize on one surface of the electrolyte layer and under the impetus of an electrical potential, will be transported through the electrolyte layer to the opposite side where the oxygen ions will recombine into molecular oxygen. Typical materials that are used to form the electrolyte layer are yttrium stabilized zirconia and gadolinium doped ceria. The electrical potential is applied to the electrolyte by way of a cathode and anode electrodes. The oxygen ionizes at the cathode and the oxygen ions recombine at the anode. Typically, electrodes can be made of mixtures of the electrolyte material and a conductive metal, a metal alloy or an electrically conductive perovskite.
In order to distribute current to the electrodes, current collectors are utilized in the form of layers on the electrodes opposite to the electrolyte. Typical current collectors in the art have included conductive metals and metal alloys, such as silver as well as mixtures of such metals and metallic oxides.
U.S. Pat. No. 5,547,494 discloses the use of such electrically driven oxygen ion transport elements in the purification of a feed stream by separating oxygen from the feed stream. In a particular embodiment shown in FIG. 7, there are two process stages placed in series. The initial stage has more process modules than the second stage. However, in each stage the process modules are connected in parallel with respect to the flow of the gas. Fewer modules are provided in a second stage because less oxygen is present in the feed gas after having passed through the first stage. In each stage, all the process modules are electrically connected in series. Each of the stages is separately powered. The series connection consumes less power than had each of the process modules then connected in parallel. Moreover, if a common power source were used for both of the stages, the second downstream stage having less oxygen to separate would be overpowered resulting in the downstream oxygen separation elements potentially being damaged. The problem with the type of arrangement illustrated in this patent is that each of the modules is a separately enclosed structure. Since the electrically driven oxygen ion transport elements must be heated to properly function, power consumed in electrically heating the modules becomes a significant expense. Moreover, in purification applications since there is a low concentration of oxygen in the second stage, oxygen molecules that are more remote from the cathode electrodes of the oxygen ion separation elements will never be separated.
As will be discussed, the present invention, among other advantages, provides a staged purification method that is far more efficient than the apparatus of the prior art in that turbulent flow is induced in the downstream oxygen ion transport elements to ensure that more oxygen will contact the element to be separated from the stream to be purified. Moreover, a much simpler arrangement of elements is provided that can be heated more efficiently than module-like elements of the prior art. In this regard, a juncture between the ceramic elements and a metallic element is provided that makes fabrication of an apparatus in accordance with the present invention much more cost effective than fabrication techniques of the prior art.